1. Technical Field
The present invention relates to a laminate made of fibrous layers for use in absorbent articles, such as sanitary towels, nappies or the like. The laminate includes an outer layer made of non-woven fabric, which is in contact with the wearer during use of the article, and an inner layer. The two layers may be interconnected in a first bonding pattern consisting of separate bonding points.
2. Background Art
Surface material for absorbent articles, such as sanitary towels, nappies or the like, means the layer which is located closest to the wearer and is in contact with the body of the wearer during use of the article. A surface material has many functional requirements, some of which are conflicting. The surface material is to feel soft and flexible to the wearer but must also be strong so as to withstand wear. It also should allow bodily fluid to rapidly pass through to the underlying absorbent body. The surface material also should prevent liquid which has been absorbed into the absorbent body from back-wetting the wearer.
Conventional surface materials in sanitary towels and nappies are non-woven fabrics, which are commercially available in a great many variants. Depending on the selection of fibers to be included in the non-woven material and the addition of any surface-active agents, such as wetting agents, the surface material is either hydrophobic or hydrophilic. Also, the liquid permeability can be controlled by varying the degree of hydrophilicity.
U.S. Pat. No. 4,333,979 describes a surface material made of non-woven fabric, which is formed from thermoplastic fibers bonded in a pattern, and is embossed so as to provide the surface material with increased thickness, softness, bulk and strength. The non-woven material consists of what is known as spunbond with separate melt-bonding points lying closely together in a pattern of separate embossings. The material in question is stated to be a very effective surface material in disposable absorbent products, such as nappies, sanitary towels and the like. The production process entails a material web being bonded by passing it through a roller nip between a heated roller pair, one of the rollers consisting of a pattern roller for forming said melt-bonding points. The production process also comprises permanent embossing of said embossing pattern in a roller nip formed by two heated, mutually matching embossing rollers. U.S. Pat. No. 4,333,979 therefore describes a material web consisting of a single layer which, as mentioned above, is double-embossed, on the one hand for bonding the web and on the other hand for creating thickness and bulk. The material has very good strength properties, but at the expense of softness and flexibility, which are worse owing to the double embossing.
Perforated non-woven fabric, that is to say fabric in which holes have been made so as to increase the liquid flow capacity, has also been available for a long time. An example of a perforated non-woven fabric is described in EP 235 309. The perforated non-woven material according to said publication consists of what is known as spunlace material with a high content of hydrophobic fibers. In a spunlace process, holes are formed in the material by means of water jets, which are sprayed against the material at high pressure. According to the publication, the spunlace material constitutes one of two layers forming the surface material and is intended to constitute the layer which is located closest to the wearer during use of the article. The aim is that the liquid be conducted through the holes and into the underlying layer. The spunlace material has a higher content of hydrophobic fibers than the underlying layer in the laminate. The fibers in the upper spunlace layer consist of 70% hydrophobic fibers and 30% hydrophilic fibers, while the underlying material layer consists of equal parts of hydrophobic and hydrophilic fibers. The underlying layer therefore has the capacity to drain liquid from the upper layer.
One problem with the material described in EP 235 309, however, is that holes which are formed by water jets are irregular in terms of both size and shape and have fibers which protrude from the edges of the holes and into the holes. These protruding fibers reduce the areas of the holes and capillary action draws the liquid into the material between the holes. The protruding fibre ends and the irregular shape and size of the holes considerably increase the risk of liquid remaining in the surface layer after wetting. Because even a small quantity of liquid on the surface material is sufficient to create a wet or soiled feeling for the wearer, this constitutes a major disadvantage of the material described in EP 235 309.
A similar material is described in EP 272 683 which also describes a surface material made of perforated non-woven fabric. In the disclosed fabric there are relatively loose fibers that are formed by perforating the non-woven material close to the holes that are intended to function as ducts for transporting liquid down to an underlying non-woven layer of what is known as the meltblown type.
As long as the fibers in the perforated layer are arranged in such a manner that they conduct liquid down to an underlying layer, the surface material functions properly. However, it is a well-known fact that a non-woven material consists of irregularly shaped fibers, which are difficult to arrange in any particular direction. This means that fibers which are intended to transport liquid down to an underlying ply will also spread liquid over the surface of the non-woven material. Therefore, some of the liquid will remain in the surface material after wetting, and the article with the surface material in question will feel wet and unpleasant against the body of the wearer.
Another problem with the surface material, as described above is the difficulty of obtaining a well-defined hole size. From EP 409 535, for example, it is well-known that the size of the holes in the perforated material is critical for obtaining optimum liquid-permeability. In the case of a non-woven material, which has some areas with a dense fibrous structure and other areas with a less dense structure, it is difficult to achieve a uniform hole size throughout the material. This is due to the fact that holes in denser fibrous areas are smaller because they are surrounded by more fibers.
Furthermore, perforated non-woven materials in such previously known surface materials have a relatively low tensile strength because the hole-manufacturing process weakens the material. As strength is important to minimize the risk of the material breaking during manufacture or use, the low tensile strength is of course a major problem.
In EP 214 608, the holes are made in the non-woven material by means of hot needles, which heat the material to a temperature just below the melting point of the material. The holes made in the material therefore have a condensed portion of the fibrous material around each hole. The problems of varying hole size and weakening of the material are thus eliminated to a certain extent. On the other hand, the problem of liquid being able to spread in the surface material and remain in the fibrous structure persists. The denser structure around the holes is also intended to draw liquid down into underlying layers, but there is an obvious risk of liquid remaining in the dense hydrophilic fibrous structure around the holes. There is also a risk of liquid spreading in the capillaries of the non-woven material and as the non-woven layer is in direct contact with the wearer, this represents a major disadvantage. Another problem is that, because portions around the holes are melted, the surface material is stiff and uncomfortable for the wearer compared with material without condensed and partly melted portions around the holes.
In WO 9740793, it has been proposed, in conjunction with the hole-punching, to seal the area around each hole so as to reduce liquid spread in the lateral direction, each hole being surrounded by an essentially liquidtight edge. In this case, however, the problem remains that the material may feel relatively stiff and uncomfortable to the wearer in comparison to a similar material without these seals around the holes.
In addition to the use of non-woven fabrics, good results have been obtained in recent years using perforated films, that is to say plastic films with a large number of small holes which are designed to allow liquid through in one direction and prevent or reduce flow in the opposite direction. Good results in terms of low back-wetting have been achieved using such materials. An early example of such a film material is described in U.S. Pat. No. 3,929,135. A later example of perforated plastic film is described in U.S. Pat. No. 6,025,049. One disadvantage of perforated plastic films is that they have a plastic feel, which troubles many wearers who want softer material with a fibrous and textile feel.
It has been difficult to provide all the desired properties of a surface material in a single layer, and it is therefore now common to have surface materials in the form of laminates consisting of two or more layers.
In order to achieve more rapid admission of liquid into the absorbent core of an absorbent product, and to create an insulating layer between the skin of the wearer and the absorbent core so as to prevent or at least reduce back-wetting, the surface material is now commonly combined with an underlying fibrous layer. An example of such a material combination is described in U.S. Pat. No. 4,761,322. Another example is described in U.S. Pat. No. 4,798,603, where a surface layer is supplemented by an underlying transport layer, said transport layer having a pore size which is smaller than the pore size in the surface layer. The latter publication states that such a material combination results in a higher liquid penetration rate and considerably lower back-wetting than a surface layer made only of, for example, spunbond. The surface layer in the absorbent article according to U.S. Pat. No. 4,798,603 can be perforated for better liquid penetration.
In the case of material combinations consisting of two or more layers closest to the wearer, it is important that the layers make good contact with one another so that insulating gaps do not occur between the outer and inner layers, because such a gap would constitute an impediment to liquid passing from one layer to the other. It is therefore necessary to bond the two layers together to form a laminate. This can be effected using bonding agent or by thermal bonding, for example ultrasonic bonding.
The bonding together is often carried out at separate places in a bonding pattern, on the one hand in order not to constitute an impediment to liquid flow, and on the other hand in order that the laminate is not too stiff. Compared with a laminate without a bonding pattern, however, the laminate is relatively stiff and feels hard and uncomfortable to a wearer.